The present invention relates to a stress reduction interposer and, more specifically, to a stress reductions interposer for a ceramic quad-flat-pack-no-lead (CQFN) surface mount device (SMD).
An SMD having a CQFN configuration typically includes a frame and lid formed of alumina or another similar material with a Copper Molybdenum (CuMo) heat sink, multiple terminal pads at the CQFN perimeter and one large ground plane in the middle of the CQFN bottom surface. This CQFN configuration is configured to be directly soldered onto a PWB but may have a relatively low solder joint thermal fatigue life due to stresses induced by coefficient of thermal expansion (CTE) mismatches in the CQFN assembly since the CQFN assembly has no compliant leads available to reduce CTE mismatch induced loads.
Previously, leaded ceramic carriers have been designed to have the CQFN soldered onto them and then to be soldered onto the PWB through the compliant leads to reduce the CTE mismatch induced loads. However, a relatively large footprint for this process is necessary. Other strategies have involved the use of compliant interposers for stress relief while still other strategies have employed constrained-core PWBs having a lower CTE or/and a specialized heatsink attached onto the PWB in order to reduce the CTE mismatch induced load. Both of these latter techniques tend to increase the costs and delivery schedules, however, and may not be applicable for larger size CQFN devices anyway.
Still other strategies have relied on the use of packages having extended leads, which would result in a larger foot print, attaching solder balls/columns onto CQFN terminal pads that is then soldered onto the PWB, which presents concerns/risks of solder joint bridging and misalignment due a large ground plane in the middle of the CQFN bottom surface and underfilling the CQFN with selective underfilled materials. In the latter case, underfilled integrity/void presents a risk/concern due to the large ground plane in CQFN. In addition, the reworkability in the underfilled CQFN would be a concern due to the risk of damage to the PWB solder pad or the use of non-reworkable underfilled materials.